BUG: bar plot can now handle bottom and left kw properly

Actually not a bug, but allows `bar` and `barh` plot to accept `bottom` and `left` kw respectively to specify the starting point.

Currently, passing `bottom` or `left` keyword results in `TypeError`
```
import pandas as pd
import numpy as np
df = pd.DataFrame(np.random.randn(5, 5))
df.plot(kind='bar', bottom=1, ax=axes[0])
# TypeError: bar() got multiple values for keyword argument 'bottom'
```

### After fix
```
import matplotlib.pyplot as plt
fig, axes = plt.subplots(1, 2)

df = pd.DataFrame(np.random.randn(5, 5))
df.plot(kind='bar', bottom=1, ax=axes[0])

df2 = pd.DataFrame(np.random.rand(5, 5))
df2.plot(kind='barh', left=[1, 2, 3, 4, 5], stacked=True, ax=axes[1])
```
![figure_1](https://cloud.githubusercontent.com/assets/1696302/3074746/24f8bebe-e351-11e3-9506-ac7de60db7fd.png)